The invention is related to optical scanning systems using a high speed rotating optical element and more specifically, a rotating optical element that causes turbulent airflow, thermal effects, and pressure disturbances in its surrounding area, resulting in optical distortions and acoustical noise problems.
Optical scanners, such as those used in film recorders, operate at very high scan rates, causing the optical deflector to rotate through the air at very high speeds. The Reynolds number for a typical deflector rotating at 27,000 rpm approaches 60,000. The airflow therefore is in the turbulent regime and consequently optical and acoustical problems arise. The optical deflector generates heat from shearing the air and in a turbulent air flow regime, heat transfer occurs via macroscopic pockets of air at differing temperatures, resulting in non-uniform air temperature. This circumstance can cause optical errors since the refractive index of air changes with temperature change. This causes wavefront errors in the deflected beam which degrade the writing spot of the scanned beam at the image recording surface.
As the geometry of the optical deflector is seldom optimum for rotation, the spinning of the optical deflector often produces an offensive acoustic noise due to the pressure disturbances caused by the irregularly shaped deflector moving through the air. As the rotational rate of the optical scanner increases, the airflow around the optical deflector becomes increasingly turbulent, causing increased optical distortion and acoustical noise.
Another problem encountered in optical scanners is the fouling of the optical deflector by airborne contaminants. During operation, optical scanners tend to draw air in toward the spin axis of the optical deflector and discharge air out radially from the spinning optical deflector. Particles in the flowing air can both foul and corrode the mirror surface of the optical deflector over time, reducing the quality of the scanned beam.
Accordingly it is an object of the present invention to enclose a rotating optical deflector with a fixed (non-rotating) shield to reduce the airflow surrounding the optical deflector and thereby improve the image quality of a scanning system employing such a rotating optical deflector.
It is another object of the invention to reduce acoustic noise associated with a high speed rotating optical deflector.
It is another object of the invention to prevent contamination of an optical deflector by airborne contaminants and prevent corrosion of the deflector surface.
It is another object of the invention to equalize the air temperature surrounding the optical deflector.
It is another object of the invention to prevent reflections from an imaging surface from inadvertently being deflected by the optical deflector back to the imaging surface and causing unintentional imaging errors.